A microscopic image capturing apparatus, having a sample setting stage that is movable along a plane (scan plane) orthogonal to an optical axis of an objective lens, has been known from before. Such a microscopic image capturing apparatus successively captures localized microscopic images of a sample surface while moving the sample setting stage, on which the sample is set, along the scan plane. In this process, a focal point of an imaging unit is finely adjusted to be successively set on desired imaging positions on the sample surface in accordance with the movement of the sample setting stage.
As an example of a focusing apparatus applicable to the microscopic image capturing apparatus with the above-described structure, there is known a focusing apparatus described in Patent Document 1. The focusing apparatus described in Patent Document 1 determines numerical formula information of a virtual plane, corresponding to a sample surface, in advance from coordinate information on three arbitrary points X, Y, and Z on the sample surface, determines Z-coordinate information of an observation point, with which an inclination component of the sample surface is corrected, based on the numerical formula information, and drives an objective lens in a Z-axis direction to set a focal point on the observation point, based on the Z-coordinate information.    Patent Document 1: Japanese Patent Application Laid-Open No. H9-304703